Potentiometer



Q Patented July 2% Iddl restore POTENTIIOMETER Daniel S. W. Kelly, Milwaukee, Wis, assignor to Allen-Bradley Company, Milwaukee, 'Wia., a-

corpcration oi Wisconsin Application February 25, 1938, Serial No. 192,494

4 Claims. (Cl. 201-55) [his invention relotes to an improved form of potentiometer for use in redlo or other high firequency circuits and resides in e novel form of construction of the some wherein discontinuity in the variation of resistance characteristics of the potentiometer as the movable contact is moved is very substeutielly diminished and ct the time there is inherently embodied in the movable branch electrical path e. fixed, or relatively fixed, resistance Zrc uency attenuating megrdtude, which, outing in conjunction with capacity provided in the circuit with which the potentiometer is used, results in iiltcringout of undesirable frequency.

Que object oi this invention is to provide in the bronchitis adjustable electrical pctlr of a potentiometer a. sufiicieut resistance 50 that cepecities conveniently provided or inherently existing in high frequency circuits render such resistance eflectlve as a. filtering or attenuation resistance for undesirable frequency.

Another object of the invention is to provide a potentiometer such that when used in radio receiver circuits adjustment thereof during operatlon oi the receiver does not give rise to objectlonoble noise.

A further object of this invention is to avoid the necessity oi any external or additional parts and to provide :2. potentiometer having the 1111-- proved characteristics above noted without requiring more then u resistance element and a traveling contect.

The potentiometer of this invention may be employed generally in electron tube and high frequency rcuits, but the best mode of use for the same now known to me is in radio receiver circuits and specific forms of this invention well adapted to this use are described herein. It is intended. however, that pr'ctectionto be granted hereon be not unnecessarily restricted thereby, but that such protection extend to the limits of the inventive concept disclosed herein, limited only by the scope of the claims appended hereto. Adjustable resistors of the potentiometer type v are, emong other uses, used as volume controls the adjustable potentiometer and ordinarily filter circuits ore provided specifically for this purpose.

For example, shown in Fig. 4 is a conventional dicgirem of c combined rectifier amplifier stage of s. fit-miller type of radio receiver circuit. [is diagrcuimcticelly indicated at l--2, two oscillatory generators of different frequency, correin in case of a superheterodyne receiver l essed inaudible high frequency potentiol of t..e intermediate amplifier frequency, and the eudio frequency of the modulated signal. In accordance with the familiar principles of filter mg, both frequencies are active in plate iced 3, While til -ii, ground, and C are provided to divert as lures 9. fraction as possible of the high frequeuoy potential from R-I, which has impressed across it the main part of the rectified audio impulses. Through contactor 4, volume control is obtuined by continuous selective tapping of 13-4 to impress the desired intensity of potential fluctuation upon. the grid of the out-put stage.

In order to prevent the affecting of the grid of the out-put stage by undesired high frequency impulses, which are productive of the well-known phenomenon called hash," as well as other wellknown disturbing efiects upon the audio elements of the circuit, resistor R may be interposed, which, in conjunction with the distributed capacity indicated by C-l causes the diversion back through ground and (7-4. of such high frequency impulses in accordance with the well-known principles of filtering. There is thus an attenuation with the aid of R of undesirable high frequency, providing R is of sufllcient magnitude, runningordinarily between 30,000 ohms and 1 megohm, in. the case of an ordinary broadcast receiver with an intermediate frequency of about 465 kc. and as low or lower than 1000 ohms 1n the case of higher frequencies. In many other circuits the resistor It in direct conjunction with a. potentiometer resistance R,-l and in conjunction with inherent or external reactance will act to produce a desired frequency discrimination. In all cases this resistance is a. substantial amount and often an appreciable fraction of the overall resistance of the principal resistance element of p the potentiometer.

he potentiometer of this invention is best described by reference to the accompanying drawing, in which Fig. l is 3. side elevation in section of one form of potentiometer of this invention;

2 is o. elevation in section of another form of potentiometer of this invention;

Fig. 3 is a side view of still another form of the potentiometer of this invention; and

Fig. 4 is a diagram of an electrical circuit indieating a potentiometer with side branch circuit resistance.

It is the discovery of this invention that a potentiometer may be so constructed that within the elements composing the potentiometer itself, without the provision of any external parts, a substantial amount of resistance may be provided in the movable branching path of the potentiometer itself and at the same time a potentiometer is provided in which variation of the movable path gives rise to a diminished amount of noise.

In Fig. 1 there is shown, in section, a principal resistance element designated by the numeral 5. Element 5 may be composed of any suitable composition, such as will provide appropriate overall resistance from end to end thereof, for example, element 5 may be composed of finely divided carbon as a conducting agent dispersed in a bonding agent, such as phenolaldehyde resin, together with appropriate filler, or may be composed of any suitable substance furnishing appropriate resistance. In radio applications, for example such as have been referred to above, the overall resistance of element 5 may be of appropriate high magnitude. Fixedly attached to and making electrical contact with element 5, are fixed contacts 6, furnishing points of electrical access through which the fixed path of the potentiometer is established. Integrally molded ends of lower resistivity may be provided to improve electrical contact with fixed contacts 6, as shown by the tippling in Fig. 1, but this is not necessary. Upon the upper surface of element 5 is shown the tact resistance as the button 8 is moved, which contact surface I, over which there travels the I side branch contactor button 8. Contactor button 8 is an element of importance in a potentiometer of this invention; In the form shown in Fig. 1, it is composed of a resistance composition material such as will provide in conjunction with the contact resistance existing at the interface between button 8 and e1ement-5 a resistance inthe side branch circuit of the potentiometer which is substantially in amount as defined above and far in excess of resistance incidental to the parts 9, I0 and I2 and the contact resistance between the button 8 and the contact surface I. Such a. magnitude of resistance in the side arm elements of a potentiometer has not heretofore been provided and, in fact, the object sought in the design of potentiometers prior to this invention has been to reduce such side arm resistance to an inconsequential fraction of the overall resistance.

The potentiometer above described may be provided with any suitable arrangement for moving the contactor 8, such as a contactor arm 8 moved by a shaft carrying a knob II. Electrical connection thereto may be made through the brush and terminal I2, composed of a spring bearing against the shaft III. Low resistance material may be integrally molded with said button 8, as indicated by the stippled portions thereof,

to render contact resistance more reliable, but

this is not necessary.

Further, by reason of the use of the high resistance contact button 8 of the potentiometer of this invention, the interface between the button 8 and the element 5 may be made relatively large without diminishing the overall resistance of element 5. This large interface and the great multitude of electrical junctions between the button and the element 5 results in a net or effective conis not subject to abrupt or discontinuous variation or transients and thus noise, which is sometimes attendant upon movement of the old type potentiometer contactor, is substantially minimized.

Another form of potentiometer of this invention is illustrated in Fig. 2, wherein a principal resistance element I 3 is shown in section. Element I3 is provided with fixed electrical contacts I4, furnishing electrical access to the fixed electrical path of the potentiometer. Laterally displaced an appreciable distance from the direct electrical path, indicated roughly by the dot-anddash line I5, is a contact surface I5. Arranged for contact with surface I6 is the sliding shoe II mounted on shaft I8 against which contact brush and terminal I9 bears. The electrical paths from shoe II to contacts I4 are roughly indicated by dot-and-dash lines 20 and 2|. As stated above, surface It is laterally displaced from line I5 and is displaced a sumcient amount so that the resistances measured along the lines 20 and 2I in summation exceed the resistance measured along the line I5 by a substantial amount as defined above, so that substantial resistance is effectively in the branch path between the parts I1, I8 and I9 on the one hand and the fixed contacts II on the other hand. In a potentiometer so con structed the effective electrical paths may be roughly indicated, as shown by the conventional zig-zag resistance symbol shown in Fig. 2, in which the resistance of the branching movable path of the potentiometer is represented by the zig-zag line 22.

When the potentiometer of the form shown in Fig. 2 is,inserted in place of the potentiometer Rr-l of Fig. 4 an external resistor in the place 01' B may be dispensed with and frequency attenuation nevertheless obtained through the inherent function of the potentiometer of this invention represented diagrammatically by the zigzag line 22 in Fig. 2. Of course, a similar result is obtained in any other equivalent application in a radio or other circuit.

In Fig. 3 is shown still another form of the potentiometer of this invention. In this form a principal resistor element 23 is provided to which there is secured, in electrical contact therewith, fixed contacts 24. Upon the upper surface of element 28 there is deposited, in electrical contact therewith, a thin layer of high resistance material 25, having on its upper surface the contacting surface 26 over which sliding contact shoe 21 travels. Contact shoe" 21 is connected as described in connection with the other forms of this invention to shaft 28 which is in electrical contact with brush and terminal 29.

High resistance layer 25 is composed of appropriate material and provided with such thickness that the resistance between contact shoe 2! and element 23 is of a substantial amount as defined above, said resistance being eflectively between the parts 21, 28 and 2! on the one hand and the fixed contacts 24 on the other hand. In a potentiometer so constructed, the movable branching electrical path inherently possesses that type of resistance which in Fig. 4 is indicated by the resistor R, and upon substitution oi the potentiometer shown in Fig. 3 for the potentiometer R-l inFlg. 4, an external resistor in the place of R may be dispense with.

In the case of either potentiometer shown in Figs. 2 or 3, as well as in Fig. 1, the nolse-produc ing. or transient-producing, effect upon movement of the contact shoe, due to discontinuity or nonhomogeneity of the resistance material contained in the principal resistor is largely eliminated by reason of the bridging effect of the large side branch resistance and the advantages of all forms of the potentiometers of this invention, beside the frequency attenuating properties, include as a distinct advantage an enhanced quietness of operation upon movement of the movable contact.

What I claim as my invention is:

1. In a potentiometer suitable for use in electron tube or other high frequency circuits, the

combination comprising a body of resistance material forming a resistance element with a contact surface thereon, fixed contacts applied to said resistance element for establishment of an electrical path therethrough, an electrically contacting branch path arm relatively movable with respect to said contact surface, a stationary branch path terminal in electrical contact with said arm, and a movable contactor in electrical contact with said arm and said contact surface for creating a branch path, said contactor being supported by said arm for relative movement with respect to said contact surface, said contactor comprising a button having a predetermined resistance of substantial amount apart from and substantially in excess of resistance incidental to contact resistance and resistance of said contact arm and terminal whereby oscillating current flowing in said branch path and try-passing from said arm and terminal to said fixed contacts by reason of the capacitive relationship therebetween encounters a predetermined resistance of substantial amount in said branch path.

2. In a potentiometer, the combination comelectrically engaging the ends of said body of resistance material, a movable contactor arranged for sliding electrical contact along the contact surface of .said body of resistance material, means for moving said contactor, and 1 terminal in electrical contact with said contactor, said contactor including resistance means having a predetermined resistance of substantial amount in excess of resistance incidental to contact resistance between said contactor and said contact surface.

3. A control device comprising in combination,

a body of resistance material forming a resistance element with a contact surface thereon, fixed contacts applied to the resistance element for establishing a fixed electrical path therethrough, a movable arm engaging a terminal at one end and having a contacting surface at its other end arranged for movement along and in contact with the contact surface of the resistance element, and a predetermined resistance of substantial amount electrically and mechanically movable in coordination with the movable arm and, electrically connected between the terminal and the fixed electrical path.

'4. A control device comprising in combination, a body of resistance material forming a resistance element with a contact surface thereon, fixed contacts applied to'the resistance element for establishing a fixed electrical path therethroug-h, a movable arm engaging a terminal at one end and having a contacting surface at its other end arranged for movement along and in contact with the contact surface of the resistance element, and a resistance greater than 1,000 ohms and other than contact resistance electrically and, mechanically movable in .,coordination with the movable arm and electrically connected between the terminal and the fixed electrical path.

DANIEL S. W. KELLY. 

